[mlir] Make ArrayType shape return smallvector

Previously the ArrayRef returned was flagged as returning address of local temporary object. This is a little bit surprising that this works given different return type difference etc and may get reworked (beyond this seemingly satisfying ubsan).

PiperOrigin-RevId: 698466497

xls/contrib/mlir/IR/xls_ops.td

@@ -55,7 +55,9 @@ def Xls_ArrayType : Xls_Type<"Array", [ShapedTypeInterface]> {
   ];
   let extraClassDeclaration = [{
     constexpr bool hasRank() { return true; }
-    ::llvm::ArrayRef<int64_t> getShape() const { return getNumElements(); }
+    ::llvm::SmallVector<int64_t> getShape() const {
+      return ::llvm::SmallVector<int64_t>(1, getNumElements());
+    }
     ::mlir::ShapedType cloneWith(std::optional<llvm::ArrayRef<int64_t>> shape,
                                  Type elementType) {
       assert(0 && "ArrayType::cloneWith unimplemented!");